It is known that in many manufacturing processes requiring heating of materials being treated, an on-line, point-by-point monitoring of material viscosity would be desirable, as deviations from a desired viscosity profile could alter the characteristics of the final material. In particular, in drawing optical fibers preforms which comprise layers with non-uniform composition (for instance because of different dopant concentrations), it is important to monitor viscosity throughout the entire preform cross-section, in the softening region of the preform.
The literature does not disclose techniques for on-line direct monitoring of viscosity of a body, in particular a body which is in fluid condition or whose behavior can be treated as that of a fluid (e.g. a glass). Viscosity information could be contained for instance from temperature values, measured by means of pyrometers. Yet temperature information supplied by a pyrometer cannot be used to obtain viscosity when the latter is to be determined near the softening temperature. In effect, considering for instance a vitreous body, like an optical fiber preform, viscosity variations of the order of 10.sup.6 occur within an interval of few degrees centigrade. The precise determination of viscosity would require measurement of temperature variations of the order of 1.degree. C. in a body which is for instance at a temperature of about 2000.degree. C., and industrial pyrometers do not have such sensitivity.